Snap zero-angle ratio factor correctly

When the dogleg angle is zero (no change between two survey stations)
there is no dampening effect and the ratio factor should be one 1, but
the divide-by-zero must be handled. This already happens in practice for
very small angles [1] but the implementation was buggy and applied the 1
to the dogleg, not the rf.

[1]
>>> def rf(x): return (2 * np.tan(x/2))/x
>>> rf(1e-10)
1.0

wellpathpy/mincurve.py

@@ -38,9 +38,16 @@ def minimum_curvature_inner(md, inc, azi):
     upper, lower = dv[:-1], dv[1:]
     dogleg = angle_between(upper, lower)
 
-    # ratio factor, correct for dogleg == 0 values to avoid divide-by-zero
-    dogleg[dogleg == 0] = 1.0
-    rf = 2 / dogleg * np.tan(dogleg / 2)
+    # ratio factor, correct for dogleg == 0 values to avoid divide-by-zero.
+    # While undefined for dl = 0 it reasonably evaluates to 1:
+    #   >>> def rf(x): return (2 * np.tan(x/2))/x
+    #   >>> rf(1e-10)
+    #   1.0
+    z  = np.where(dogleg == 0)
+    nz = np.where(dogleg != 0)
+    rf = 2 * np.tan(dogleg / 2)
+    rf[nz] /= dogleg[nz]
+    rf[z] = 1
 
     md_diff  = md[1:] - md[:-1]
     halfmd   = md_diff / 2

wellpathpy/test/test_position_log.py

@@ -39,6 +39,34 @@ def test_copy():
     original.depth += 10
     np.testing.assert_equal([1,2,3,4], copy.depth)
 
+def test_straight_down_segment_preserves_depth():
+    """
+    When the well is straight down, the vertical depth and measured depth
+    should increase at the same rate.
+    """
+    md  = [0, 3, 7]
+    inc = [0, 0, 0]
+    azi = [0, 0, 0]
+
+    pos = deviation(md, inc, azi).minimum_curvature()
+    np.testing.assert_array_equal(md, pos.depth)
+    np.testing.assert_array_equal([0, 0, 0], pos.northing)
+    np.testing.assert_array_equal([0, 0, 0], pos.easting)
+
+def test_straight_nonvertical_segment():
+    """
+    The well has a constant inclination which means delta(md) > delta(vd), the
+    path is not straight down. This doesn't test for the path going back up.
+    """
+    md  = np.array([1,  3,  7])
+    inc = np.array([30, 30, 30])
+    azi = np.array([0, 0, 0])
+
+    pos = deviation(md, inc, azi).minimum_curvature()
+    delta_md = md[1:] - md[:-1]
+    delta_vd = pos.depth[1:] - pos.depth[:-1]
+    assert (delta_md > delta_vd).all()
+
 @pytest.mark.xfail(strict = True)
 def test_straight_hole():
     md = np.array([0, 10, 20]).reshape(3, 1)